The present invention relates to a light-emitting device, an emissive type display having a plurality of the light-emitting devices arranged in matrix and controlling their light emitting operation for displaying an image, and an illumination device using the light-emitting devices.
An organic light-emitting diode (OLED) is a device which, when its emissive layer formed of an organic thin film is injected with holes and electrons, transforms an electric energy into a light energy to emit light. Unlike a non-emissive type display as represented by a liquid crystal display, an emissive type display having the organic light-emitting diodes as light-emitting devices (referred to as “OLED display”) is self-luminous and requires no auxiliary light source such as a backlight and therefore has an advantage of being thin and lightweight. The OLED display is characterized by a wide view angle and a high response speed.
The OLED has a substrate deposited successively with an optically transparent electrode, a hole transporting layer, an emissive layer, an electron transporting layer and a reflective electrode formed of a light reflecting metal. Applying a DC voltage between the transparent electrode and the reflective electrode causes an emission of light of a predetermined wavelength from the emissive layer. A part of light emitted from the emissive layer passes through the transparent electrode to be seen by an observer. At this time, since rays emitted almost parallel to the surfaces of laminated layers and those with incidence angles at the interfaces of layers greater than a critical angle propagate in a direction parallel to the laminated layer surfaces and not toward the observer, they are not put to effective use as display light.
Generally, an external coupling efficiency (a ratio of a quantity of light leaving the device toward an observer to a quantity of light emitted from the emissive layer) is said to be around 20% according to an estimation by classical optics. A large part of the light emitted from the emissive layer propagates in a direction parallel to the laminated layer surfaces and is lost. Thus, to realize an OLED display with high brightness and low power consumption, it is important to reduce a propagation loss and improve the external coupling efficiency.
To meet this requirement, JP-A-2002-278477 discloses an active matrix light-emitting device which comprises an emissive layer formed in a matrix shape, a transparent conductive layer to supply electrons or holes to the emissive layer, and a TFT substrate formed with a matrix control circuit for controlling an emission of the emissive layer. The active matrix light-emitting device is characterized by a low refractive index layer provided on a surface of the transparent conductive layer on an opposite side of the emissive layer and which has a refractive index of 1.01 to 1.3.
Japanese Patent No. 2991183 and JP-A-2003-163075 disclose an organic electroluminescence element that has one or more organic layers including an emissive layer between an anode and a cathode and which also has a grating or zone plate formed in a position that prevents a total reflection on an interface of the element.
However, a low-refractive index layer has a low refractive index that common materials cannot achieve and which in JP-A-2002-278477 could only be realized by the use of a special material such as silica aerogel. A fabrication of silica aerogel requires a special method and is difficult to accomplish.
Where a conventional grating is used, the quantity of light changes depending on the wavelength and visual angle because light, that would pass through a device interface if no grating was provided, is refracted by the grating. Furthermore, ambient light is also refracted, so a polarization of light refracted by the grating changes, giving rise to a problem of reduced contrast.
An object of this invention is to improve an external coupling efficiency to efficiently extract to the outside light emitted from the emissive layer by using a simple manufacturing method.
Another object of this invention is to provide an emissive type display (OLED display) and an illumination device both of which minimize a reduction in contrast and improve the external coupling efficiency.